Major histocompatibility (MHC) Class I gene activation occurs during embryonic development, so that at birth most of somatic cells express the antigens derived from both paternal and the maternal MHC genes. The precise timing, and the types of Class I antigens expressed during embryonic development have been so far controversial. Class I antigen expression during fetal development poses another unsolved immunological question, i.e. even though the fetus carry histoincompatible paternal Class I antigens, allograft rejection is not detected during pregnancy. To determine Class I antigen expression in embryogenesis we studied cell surface expression of Class I antigens throughout mouse gestation. Two series of monoclonal antibodies, one reacting with all types of Class I antigen (rat xeno antibodies), another reacting with classic, polymorphic H-2 antigens (mouse alloantibodies) were employed. A significant antibody binding was noted in embryos at gestation day 10 (somite stage) and after. No binding was detectable in earlier embryos. Thus, the time of the antigen expression is later than previously indicated. Only monoclonals reacting with all types of Class I antigens were positive on day 10 embryos. Studies on the onset of Class I gene transcription and that of the protein synthesis are underway. Studies of the effect of anti-paternal Class I antibodies (often produced in pregnancy) on the fetal immune development have been continued; radiolabeled monoclonal antibodies were injected into pregnant mice, radioactivity was found in various fetal tissues, much of which represented intact Ig as tested by SDS-gel electrophoresis, indicating active passage of the antibodies to the fetuses. Class I antigen expression and the immune function of the animals treated with the antibodies are being studied. To elucidate T cell activity in pregnancy, the ability of generating cytotoxic T cells in multiparous mice was examined. We found that during pregnancy T cell reactivity is markedly increased in allopregnant animals, but not in syngeneic matings indicating maternal allosensitization.